Redox flow cells are secondary batteries in which all electrochemical components are dissolved in the electrolyte.
The energy capacity of the redox flow cell is independent of its power, as the energy available only depends on the electrolyte volume (amount of liquid electrolyte), whereas the power depends on the surface area of the electrodes.
A well-established example is the vanadium redox flow battery, which contains redox couples entirely based on vanadium cations (see FIG. 1). Typical performance data are shown in Tab. 1:
TABLE 1Vanadium redox battery performance.Specific Energy10-20 Wh/kg (36-72 J/g)Energy Density15-25 Wh/L (54-65 kJ/L)Charge/Discharge Efficiency75-80%<Time Durability10-20 yearsCycle Durability>10,000 cyclesNominal Cell Voltage1.15-1.55 V
The following half-cell reactions occur in all-vanadium flow cells:

Although the vanadium redox flow battery is well established, there are a wide range of less commonly used inorganic flow cell chemistries, including the polysulfide-bromide battery (PSB):

The wide-scale utilization of flow batteries is presently limited by the availability and cost of the redox materials, in particular those that are based on redox-active transition metals such as vanadium, and/or require precious-metal electrocatalysts.
A metal-free organic-inorganic aqueous flow battery which combines a quinone/hydroquinone redox couple with a Br2/Br− redox couple has been recently proposed by Huskinson et al., Nature 505, 195-198. Herein, bromine is used herein as oxidiser, in combination with the reduced (hydroquinone) form of 9,10-anthraquinone-2,7-disulphonic acid acting as reductant.
However, the toxicity of inorganic redox materials such as vanadium salts or bromine limits the applicability of flow batteries for energy storage in the context of distributed, modular energy generation technologies that use (intermittent) “green power” such as wind, photovoltaic, or hydroelectric power.
In view of the above, the development of new organic redox materials, which offer the prospects of low material costs and reduced toxicity of the energy storage materials would be desirable.
The simplest form of a battery requires a single-electron redox process on both sides of an ion-permeable membrane. This can also be done with organic molecules through the use of stabilised radicals, but in general organic radicals are either relatively chemically reactive or require so much stabilisation that it is difficult to generate the high redox potentials required for a significant cell voltage.
While single-electron transfer materials (e.g. triazine) have been considered, they involve limitations in that they only comprise one electron per molecule.
Molecules such as the quinones have two electron transfers and can therefore have twice the energy density (per mass/volume/cost). Moreover, two-electron processes prevent the necessity of forming unpaired electrons (radicals) in organic molecules, and can readily be accommodated by changing the α- and π-bonds into a different oxidation state. Two-electron processes can therefore be advantageous for enhancing both the stability and the cell potential of a battery system.
To achieve high potentials it is desired that the charged battery should contain a reduced compound and an oxidised compound that are both relatively reactive compared to the uncharged system. It is this electrochemical reaction between the two materials that drives the battery process and the flow of electrons around the external circuit. As long as side reactions do not occur that reduce the lifetime of the compounds, it is preferable to have the highest compound reactivity possible to thereby enable the highest cell potential.
One solution is to take two similar moieties—for example, two different quinones (see e.g. WO 2014/052682 A2)—that have different redox potentials. However, this process makes it difficult to produce high cell potentials due to the relative similarity of the compounds—within one class it is unlikely that both the oxidised form of one and the reduced form of the other will have high reactivity. Therefore, the generally preferred approach is to optimise both molecules independently to achieve the desired high reactivity.
However, leakage of the redox materials through the separator membrane has been identified as being a cause of device degradation, as the available volume of redox active material is slowly reduced and there is also the potential for unwanted reactions to occur between the different redox active materials.
In view of the above, there remains a need for novel organic electroactive redox materials which are readily available and exhibit reduced toxicity and excellent energy density. Moreover, it is desirable to provide organic flow cell batteries that have a high operating potential, high cell output voltage, long lifetime and that may be produced at favourably low costs.